The phrase _invertebrate animals_ originated with Lamarck, and it expresses, as Cuvier remarks, perhaps the only circumstances in their organization which is common to them all. They were previously known as _white-blooded animals_, a designation which was soon shown to be improper, by the discovery that an entire class (the _annelides_) possesses red blood. The system of Linnæus and Bruguière formed the basis of his course when he first began to lecture on the subject; he subsequently adopted a new classification, founded on their anatomy, which had been published in 1795. This he afterwards modified in various ways, as new discoveries were made, and as new relations suggested themselves to him. In his system of invertebrata, forming an octavo volume, published in 1810, he adopted the class of _crustacea_, and created that of _arachnides_, a step which he judged necessary, in consequence of some new information that had been communicated to him on the heart and pulmonary sacs of spiders. In a previous work he had admitted the _annelides_ to the rank of a separate class, in consequence of Baron Cuvier’s observations on their circulating organs and the colour of their blood, which resembles that of the vertebrate races. Two other classes were created, in his “Philosophical Zoology,” viz. the _infusoria_ and the _centipedes_; and in this work also he first deviated from the ordinary practice of commencing the arrangement with the most perfectly organized, the inverse order being more in accordance with his theoretical views, which assumed a gradual _progression_ in the composition of animal organs, proceeding from the most simple to the most complex. It was in a small volume, entitled “An Extract from the Zoological Course in the Museum of Natural History, on the invertebrate Animals,” &c. published in 1812, that he first presented his general distribution of animals into three grand divisions, _apathetic_, _sentient_, and _intelligent_. This plan he made the foundation of his great work, and the method in which he applied it, as well as his ideas regarding the constitution of the different classes, and their relation to each other, will be understood from the subjoined table. Invertebrate Animals. I. APATHETIC ANIMALS. 1. _Infusoria._ 2. _Polypes._ 3. _Radiarii._ 4. _Vermes._ (_Epizoaires._) II. SENTIENT ANIMALS. 5. _Insecta._ 6. _Arachnides._ 7. _Crustacea._ 8. _Annelides._ 9. _Cirrhipedes._ 10. _Mollusca._ Vertebrate Animals. III. INTELLIGENT ANIMALS. 11. _Pisces._ 12. _Reptilia._ 13. _Aves._ 14. _Mammiferæ._